1. Field of the Invention
The present invention relates to a radar system mounted on a vehicle, which is suitable for use as a car-to-car distance measuring system for a vehicle.
2. Description of the Related Art
This type of known radar system may be an FMCW (Frequency Modulated Continuous Wave) radar system downsized by use of a transmit-receive shared antenna, of which a mountability on an automobile is thereby enhanced.
FIG. 5 is a block diagram showing a configuration of the conventional FMCW radar system.
Referring to FIG. 5, there are shown an oscillator 1, a power divider 2, a transmitting amplifier 3, a circulator 4, a transmit-receive shared antenna 5 including a horn antenna 51 and a reflection mirror antenna 52, a target object 6, a receiving amplifier 7, a mixer 8, a filter 9, an AGC (Automatic Gain Control) amplifier 10, an AD (Analog-to-Digital) converter 11, a signal processor 12, an antenna scan motor 13 for deflecting a transmitting direction and a receiving direction of the transmit-receive shared antenna 5 on the basis of an output given from the signal processor 12, and an handle angle sensor 14.
Next, an operation of the thus constructed prior art radar system mounted on the vehicle will be explained.
The signal processor 12 outputs a linear voltage signal for a frequency modulation. With this frequency modulation voltage signal, the oscillator 1 generates frequency-modulated electromagnetic waves. The power divider 2 divides the electromagnetic wave into two groups of electromagnetic waves. One group of electromagnetic waves are inputted to the mixer 8, while the other group of electromagnetic waves, after being amplified by the transmitting amplifier 3, arrive at the transmit-receive shared antenna 5 via the circulator 4 and are outputted into the air space from this antenna 5.
The electromagnetic waves outputted into the air space from the transmit-receive shared antenna 5, are reflected by the target object 6 and inputted back to the transmit-receive shared antenna 5 with a delay time Td with respect to the transmitting electromagnetic waves. Further, if the target object 6 has a relative velocity, the receiving electromagnetic waves are inputted to the transmit-receive shared antenna 5 with a Doppler shift Fd with respect to the transmitting electromagnetic waves. The electromagnetic waves received by the transmit-receive shared antenna 5 are, after being amplified by the receiving amplifier 7, mixed with the transmitting electromagnetic waves by the mixer 8, thereby outputting beat signals corresponding to the delay time Td and the Doppler shift Fd. The obtained beat signals are transmitted through the filter 9 and, after being amplified by the AGC amplifier 10, inputted to the AD converter 11. The signal processor 12 calculates a relative velocity and a relative distance to the target object 6 from the beat signals.
Next, a method by which the signal processor 12 calculates the relative velocity and the relative distance to the target object 6, will be put into discussion.
FIG. 6 is an explanatory diagram showing one example of calculating the relative distance and the relative velocity by use of the radar system mounted on the vehicle.
Referring to FIG. 6, the transmitting signal is frequency-modulated with a frequency sweep bandwidth B at a modulation cycle Tm. The receiving signal has the delay time Td till the transmitting signal is inputted to the transmit-receive shared antenna 5 since the transmitting signals have been reflected by the target object 6 existing at, e.g., a distance R. Further, if the target object 6 has a relative velocity, the receiving signal is Doppler-shifted by a frequency fd with respect to the transmitting signal.
In this case, the mixer 8 outputs, as beat signals, a frequency difference Fbu between the transmitting signal and the receiving signal when the frequency rises, and a frequency difference Fbd between the transmitting signal and the receiving signal when the frequency lowers. The signal processor 12 takes in those beat signal as pieces of data via the A/D converter 11, and executes an FFT (Fast Fourier Transformation) process on these pieces of data, thereby obtaining the frequency differences Fbu, Fbd and receiving intensities Mu, Md. The receiving intensities Mu, Md of the frequency differences Fbu, Fbd are generally the same, and hence an average value M thereof is given such as M=Mu=Md.
The relative distance R and the relative velocity V of the target object 6 are given by the following formula                               R          =                                    TmC                              4                ⁢                B                                      ⁢                          (                              Fbu                +                Fbd                            )                                      ,                  V          =                                    λ              4                        ⁢                          (                              Fbu                -                Fbd                            )                                                          (        1        )            
where Fbu, Fbd are the frequency differences, B is the frequency sweep bandwidth, Tm is the modulation cycle, C is the light velocity (=3.0xc3x97108 m/s), and xcex is the wavelength of the carrier wave (xcex=4.0xc3x9710xe2x88x923 m, if the carrier wave basic frequency Fo=77 GHz).
Further, if a plurality of target objects 6 exist, the frequency differences Fbu, Fbd of the same object are elected from the plurality of frequency differences Fbu between the transmitting signals and the receiving signals when the frequencies rise, and the plurality of frequency differences Fbd between the transmitting signals and the receiving signals when the frequencies lower, and the relative distance R and the relative velocity V are obtained from the formula (1).
Given next is an explanation of a method by which the signal processor 12 calculates a direction of the target object 6 from the receiving intensity M.
For instance, Japanese Examined Patent Publication No. Hei 7-20016 discloses typical systems such as a mono-pulse system, a sequential lobing system and a conical scan system as conventional methods of calculating the direction. The sequential lobing system among those systems is herein described.
The signal processor 12 measures a relative distance, a relative velocity and a receiving intensity M1 in a predetermined direction xcex81, and thereafter operates the antenna scan motor 13 to make a shift to a next direction xcex82. The signal processor 12 similarly measures a relative distance, a relative velocity and a receiving intensity M2. Data about the same relative distance and relative velocity are chosen among pieces of detection data in the plurality of directions, and an angle can be measured basically from a relationship in magnitude between the receiving intensity M1 and the receiving intensity M2.
To be more specific, a sum pattern S(xcex8) and a difference pattern D(xcex8) are obtained from antenna beam patterns B1(xcex8), B2(xcex8) in the predetermined two directions xcex81, xcex82 by the following formulae (2) and (3):
S(xcex8)=B1(xcex8)+B2(xcex8)xe2x80x83xe2x80x83(2)
D(xcex8)=B1(xcex8)xe2x88x92B2(xcex8)xe2x80x83xe2x80x83(3)
Next, a discriminator DS(xcex8) shown in the following formula (4), which is standardized by the sum pattern S(xcex8), is obtained.
xe2x80x83DS(xcex8)=D(xcex8)/S(xcex8)xe2x80x83xe2x80x83(4)
Subsequently, the discriminator DS(xcex8) shows a relationship of simple increment or simple decrement with the measured angle value xcex8 within a half-value width xcex8s of the sum pattern S(xcex8).
An angle xcex8n at which a central angle xcex8o in the predetermined two directions xcex81, xcex82 is standardized by the half-value width xcex8s of the sum pattern S(xcex8), is given by the following formula (5). An inclination k of the discriminator DS(xcex8) in the vicinity of xcex8n=0, is given by the following formula (6).
xcex8n=(xcex8xe2x88x92xcex8o)/xcex8sxe2x80x83xe2x80x83(5)
k=DS(xcex8)/xcex8nxe2x80x83xe2x80x83(6)
Furthermore, the discriminator DS obtained by observation from the receiving intensities M1 and M2 is obtained from the following formula (7):
DS=(M1xe2x88x92M2)/(M1+M2)xe2x80x83xe2x80x83(7)
Hence, the measured angle value xcex8 can be obtained by the following formula (8) from the half-value width xcex8s, the inclination k and the angle xcex8o which can be calculated beforehand, and from the discriminator DS obtained by observation.
xcex8=xcex8s/kxc2x7DS+xcex8oxe2x80x83xe2x80x83(8)
Whether or not the target object 6 is a car traveling ahead of the self-car on the same lane, is judged based on a road curvature obtained by the handle angle sensor 14 as well as on the angle, the relative velocity and the relative distance to the target object 6 which have been measured as described above. There are carried out a car-to-car distance alarm and a chasing travel to keep a safe car-to-car distance.
When calculating the relative distance and the relative velocity in the prior art radar system mounted on the vehicle, however, if a linearity of frequency modulation in FIG. 6 is impaired, the receiving intensities M of the beat frequencies Fbu, Fbd are influenced by the linearity with an error in frequency, and therefore take the different values Mu, Md. As a consequence, an measured angle value also has an error, resulting in such a problem in terms of a system operation as to be incapable of properly recognizing a car traveling ahead and an obstacle as well.
The error might be reduced if the linearity is enhanced. The enhancement of the linearity, however, necessitates increasing a scale of the hardware and a load on signal processing. This can not be attained in terms of costs and operating conditions of an automobile.
The present invention has been made to solve the problems described above, and therefore has an object to provide a radar system mounted on a vehicle, capable of properly measuring an angle even when a linearity is impaired without increasing a scale of hardware and a load on signal processing and exhibiting a high performance at a low cost.
To accomplish the above object, according to one aspect of the present invention, a radar system mounted on a vehicle comprises a transmitting means for transmitting electromagnetic waves subjected to plural kinds of frequency modulations, a receiving means for receiving the receiving electromagnetic waves reflected by a target object, a signal processing means for calculating a relative distance and a relative velocity to the target object and a receiving intensity on the basis of a transmitting signal of the transmitting means and a receiving signal of the receiving means, and calculating a direction of the target object from the receiving intensity, and a scanning means for deflecting a transmitting direction of the transmitting means and a receiving direction of the receiving means on the basis of an output given from the signal processing means, characterized in that the signal processing means executes an angle measuring process based on a combination of the receiving intensities in respective frequency-modulated phases in the plurality of directions.
This radar system mounted on the vehicle is characterized in that the signal processing means determines an average of respective measured angle values.